In biology, monolayers of amphiphilic molecules have long been used as well defined model systems of cell membranes. In recent years, however, the interest in ultrathin organic films has dramatically increased due to their potential applications in the microelectronic industry and in the biotechnology industry. These applications call for decreased film thickness than heretofore available. Thus, improved and carefully engineered instruments are necessary for the fabrication of ultrathin films with high degree of structural order and well defined characteristics.
Historically, monomolecular films at the air-water interface are produced by using a Langmuir trough (I. Langmuir, J. Am. Chem. Soc. vol. 39, 1848, (1917)). The trough, a special form of container, is filled with a subphase, usually highly purified water. A dilute solution of an amphiphilic molecule is dissolved in a volatile solvent and is deposited on the subphase. As the solvent evaporates, a monomolecular layer of the amphiphile spreads over the surface of the subphase. This thin, quasi two-dimensional film is then compressed laterally by means of a barrier to form a solid monomolecular film. Supported planar monomolecular or multimolecular layers (monolayers or multilayers) can then be prepared by transferring the floating monolayer onto a solid substrate. The transfer is usually accomplished by use of the Langmuir-Blodgett technique (K. B. Blodgett, J. Am. Chem. Soc., vol. 57, 1007 (1935)). These supported planar membranes, called Langmuir-Blodgett films are known for their high quality and well-organized structure. However, defects in these films are present, since the commercial instruments that are currently used for their preparation are inadequate for the characterization of the physical state of the membrane prior to the transfer stage.
Recently the technique of fluorescence microscopy has been applied toward the problem of determining the structure and properties of monolayers (Peters R. and Beck K., Proc. Natl. Acad. Sci. USA, vol. 80, 7183 (1983)). In this method, the monolayer can be directly visualized at the air-water interface through a microscope objective. The technique is comprised of a specially designed monolayer trough that is positioned on the stage of a fluorescence microscope equipped with the epi-illumination technique, where a light source (e.g. a laser or lamp) excites certain fluorescent probes within the monolayer. The emitted fluorescence is then observed through the microscope objective. In order to observe the monolayer, a small amount (less than 1%) of a fluorescent amphophilic dye is mixed with the monolayer forming compound.
Fluorescence microscopic analysis of the monolayers at the air-water interface and at solid supports, has revealed that the structure and order in these ultrathin films are strongly influenced by a variety of factors that can be easily controlled for the production of films free of defects. For example, changes in temperature, surface pressure, the degree of impurities in the subphase or in the film, or the nature of the solid support itself, can alter the structure and properties of certain monolayers. (Losche M. and Mohwald H., Eur. Biophys. J., vol 11, 35, (1984); Weis R. M. and McConnell H. M., J. Phys. Chem., vol. 89, 4453 (1985); Suel M., Subramaniam S., and McConnell H. M., J. Phys. Chem., vol. 89, 3592, (1985)).
Prior to the development of this invention, there were no commercially available monolayer troughs that were suitable for fluorescence microscopy of monolayers at an air-water interface. Flow and convection of the subphase in conventional monolayer troughs disrupt visualization of the monolayer using the fluorescence microscopy technique. Carefully engineered instrumentation is necessary to overcome this problem. Moreover, due to the smaller size of the microscope troughs, the problem of leakage around the trough barriers is a far more serious problem than in the larger scale Langmuir troughs.
A number of fluorescence microscope troughs have been developed in research laboratories (Peters R. and Beck K., ibid; Losche M. and Mohwald H. Rev. Sci. Instrum. vol. 55, 1968 (1984); McConnell H. M., Tamm L. K., and Weis R. M., Proc. Natl. Acad. Sci. USA, vol 81, 3249, (1984); and Gaub H. E., Moy V. T., McConnell H. M., J. Phys. Chem. vol 90, 1721 (1986). These troughs were designed for specific applications, and their use is limited to the technology available in those laboratories. Thus, due to their lack of versatility, they are not feasible for commercial exploitation. For example, the Losche and Mohwald trough is too costly because it is permanently cemented to the microscope objective. The Peters and Beck trough and the McConnell et al. trough can only be used on upright microscopes.